1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China
2. Information System Laboratory, Department of Electrical and Computer Engineering, Seoul National University, Seoul 151-744, Republic of Korea
3. School of Information Engineering, Zhengzhou University, Zhengzhou 450001, China
4. Instituto de Telecomunicações, 1049-001 Aveiro, Portugal